1. Field of the Invention
The present invention relates to a received signal strength indicator for a zero intermediate frequency or a low intermediate frequency radio device such as a receiver or transceiver. Such a radio device receives radio frequency signals, and, when a transmit part is also present, also transmits signals. Such a radio device can be a cellular radio device, a cordless telephone, a wireless local area network radio device, a satellite radio device, or any other suitable radio device.
2. Description of the Related Art
In the U.S. Pat. No. 5,603,112, a received signal strength indicator is disclosed. The received signal strength indicator is used for performing signal strength measurements of received radio frequency signals in radio frequency receivers. The received signal strength indicator comprises analog to digital converters for sampling quadrature down modulated signals at an intermediate frequency. Output signals of the analog to digital converters are supplied to respective comparators that compare magnitudes of the quadrature intermediate frequency signals. Based on comparison results, the magnitudes of either the in-phase signal or quadrature signal is supplied to either a divide-by-16 divider, to a divide-by-8 divider, or by a divide-by-4 divider. The in-phase and quadrature signals, and the divided in-phase and quadrature signals are combined in a number of adders, of the adders providing the received signal strength indicator signal. Through such a comparison, division, and combining, a scaled combination of approximated magnitudes of in-phase and quadrature down converted quadrature signals is computed, approximating the magnitude of the received vector, the received vector being the square root of the sum of the squared magnitudes of the in-phase and quadrature down converted quadrature signals. Such a computation of an approximation of the magnitude of the received vector, used as a received signal strength indicator signal in the intermediate frequency domain, is complex.
In the U.S. Pat. No. 5,338,985, a received signal strength indicator for operation at intermediate frequencies is disclosed. The received signal strength indicator comprises a number of cascaded amplifiers that amplify an intermediate frequency signal. Output signals of the respective amplifiers are rectified, and, after voltage to current conversion, currents representing rectified signals are weightedly added so as to obtain a received signal strength indicator signal. From the last amplifier of the multistage amplifier to the first amplifier, the amplifiers successively run out of their linear region and get into their limiting or clipping range, with an increasing input signal amplitude. Herewith, the resulting received signal strength indicator signal approximates a linear function of the logarithm of the input intermediate frequency signal. The more amplifier stages in the cascade of amplifiers, the better the linear approximation is.
In the U.S. Pat. No. 5,978,664, peak detector is disclosed. The peak detector is particularly suitable for measuring the peak value of an RSSI signal formed by a multistage limiting and summing logarithmic amplifier. Reducing a ripple in a combination of a multistage limiting and summing logarithmic amplifier and detector, desirable to get a more reliable received signal strength indicator signal, renders the time response of the combination worse, i.e., the signal undesirably slowly decays after a sudden change in the amplitude of the highly dynamic radio signal to be measured.
On page 62 of the DRAFT Supplement, Part 11, of the IEEE 802.11b standard, operating channels are shown for North American Channel selection in the so-called 2.4 GHz ISM band.
It is an object of the invention to provide a received signal strength indicator signal at zero intermediate frequency or low intermediate frequency that is simple, that is low ripple, and that has a high dynamic range.
It is another object of the invention to provide a received signal strength indicator that produces a received signal strength indicator signal approximating a linear function of the logarithm of an input signal supplied to the received signal strength indicator.
It is still another object of the invention to provide a received signal strength indicator wherein for a particular modulation type of signals, such as QPSK, signal glitches are avoided.
In accordance with the invention, a method of determining a received signal strength indicator signal from an in-phase signal component and a quadrature signal component of a low intermediate frequency signal that represents a received radio frequency signal, is provided comprising:
determining a first absolute value from said in-phase signal component;
determining a second absolute value from said quadrature signal component; and
said first and second absolute values.
The approximately linear function of the logarithm of the input signal is obtained by logarithmically processing of signals, either before or after determining the first and second absolute values.
Preferably, logarithmic processing is performed before determining the first and second absolute values, by a multistage limiter followed by an adder for adding signals produced by the limiter, in both the in-phase and quadrature signal path.
By low pass filtering the added absolute values, signal glitches are avoided.